sanitycheck: make --subtest and --test mutually exclusive
sanitycheck: make --subtest and --test mutually exclusive

They're effectively mutually exclusive already because
options.sub_test in main() immediately discards any --test
argument(s). This commit preempts user confusion thanks to this new
message:

 sanitycheck: error: argument --sub-test: not allowed with
    argument -s/--test

Signed-off-by: Marc Herbert <77e7c7d746b52c6eb01119b65a031ae1f650246d@intel.com>

zephyrproject-rtos/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,ldts/zephyr,nashif/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,ldts/zephyr,ldts/zephyr,finikorg/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr